Terpolymers comprising styrene, maleic anhydride and acrylonitrile which have been modified by an elastomer have been described in several U.S. patents. One early one, U.S. Pat. No. 2,914,505, provides a blend comprising 10 to 40 parts of an elastomer of a conjugated diolefin and acrylonitrile with 60 to 90 parts of a terpolymer of the above-recited type. Such compositions are prepared either by blending of the two polymers mechanically, as on a two roll mill or Banbury mixer, or by mixing latices of each, followed by coagulation and drying of the mixture. The compositions are said to exhibit enhanced softening points and although a temperature range is not disclosed, the patentees indicate that then existing compositions had low softening points, in the region of 100.degree. C. Although the terpolymer can contain high amounts of maleic anhydride and acrylonitrile, e.g., 50 to 80% styrene, 5 to 35% maleic anhydride and 15 to 40% acrylonitrile, blending of the rubber in the manner described does not afford a uniform graft structure leading, in turn, to the most improved physical properties.
U.S. Pat. No. 3,919,354 discloses impact resistant polymers comprising rubber modified copolymers of alkenyl aromatic and unsaturated dicarboxylic anhydride monomers. Proportions reacted include 65 to 95 parts by weight of alkenyl aromatic and 35 to 5 parts by weight of the unsaturated anhydride. The amount of rubber ranges from about 5 to 35 parts per 100 parts of the total weight of monomers and rubber. The patent further discloses that comonomers polymerizable with the alkenyl aromatic monomer such as acrylonitrile can be present in a proportion of 10 to 40 weight percent of the anhydride/butadiene/styrene polymer composition.
U.S. Pat. Nos. 4,223,096 and 4,262,096, assigned to Monsanto Company, provide respectively a process for preparing rubber-modified terpolymers and a terpolymer or resin which comprises 50 to 83% monoalkenyl aromatic monomer, 15 to 30% unsaturated dicarboxylic acid anhydride and 2 to 20% unsaturated nitrile. The terpolymer is also modified with 2 to 30% rubber such as butadiene polymers and copolymers.
The process calls for the polymerization of the monomers in the presence of the rubber with a gradual continuous addition of the anhydride monomer. More specifically, a solvent polymerization is employed wherein the rubber is dissolved in a mixture of the monovinyl aromatic monomer and the unsaturated nitrile monomer and a solvent such as methyl ethyl ketone and toluene. The resulting solution is polymerized with late addition of the anhydride dissolved in the same or similar solvent, as set forth in U.S. Pat. No. 3,919,354. While the patentees reported improved physical properties over resins unmodified with rubber, they also noted that only if the content of unsaturated nitrile in the polymer were maintained below about 20 percent, a moldable composition would be obtained that could be left at 120.degree. C. for several days without becoming infusible. They found surprising that only a narrow range of compositions within the art were useful, viz., those having an unsaturated nitrile content of less than 20 percent.
The patents discussed herein have not disclosed a useful rubber-modified, thermoplastic resin containing 100 total parts by weight of an olefinically unsaturated nitrile monomer, a monovinyl aromatic monomer and an unsaturated dicarboxylic acid anhydride monomer where the nitrile monomer content is at least about 20 parts and can be higher or the anhydride monomer content is at least about 30 parts. Such resins are prepared in a simple one stage polymerization without solvents or continuous controlled addition of monomers.